Known radar systems that are currently used for distance measurement in vehicles essentially comprise two separate radar subsystems which operate in different frequency bands. For distance measurements in a short range (short range radar), radar systems currently used are typically those which operate in a frequency band around a mid-frequency of 24 GHz. Short range typically means distances from 0 to about 20 meters from the vehicle. The frequency band from 76 GHz to 77 GHz is currently used for distance measurements in the long range, that is for measurements in the range from about 20 meters to around 200 meters (long range radar). These different frequencies are an impediment to the creation of a single concept for a radar system which can carry out measurements in a plurality of range zones, and in principle result in the need for two separate radar systems.
The frequency band from 77 GHz to 81 GHz is likewise suitable for short range radar applications, and has also been made available by the authorities for this purpose, so that a frequency range from 76 GHz to 81 GHz is now available for automobile radar applications in the short range and in the long range. A single multirange radar system which carries out distance measurements in the short and in the long range using a single radio-frequency transmission/reception module (RF front-end) has, however, not yet been feasible for various reasons. One reason is that circuits which are manufactured using III/V semiconductor technologies (for example gallium-arsenide technologies) are used at the moment to construct known radar systems. Gallium-arsenide (GaAs) technologies are highly suitable for the integration of radio-frequency components, but it is generally not possible to achieve a degree of integration which is as high, for example, as that which would be possible with silicon integration because of technological restrictions. Furthermore, only a portion of the required electronics are manufactured using GaAs technology, so that a large number of different components are required to construct the overall system.
RF oscillators which are manufactured using SiGe-technology for RF front-ends (i.e., RF circuits for transmitting and receiving RF signals) and which can be tuned throughout the entire frequency range from 76 GHz to 81 GHz have, however, become possible only as a result of the latest manufacturing technologies, that allow for the production of radar systems which are substantially more compact and more cost effective compared to known radar systems. Beside a compact architecture, a large “field of view” of the radar sensor is desired when designing RF front-ends of radar systems, wherein the transmitted signal power increases with an increasing field of view.
Monostatic radar systems which have common antennas for transmitting and receiving signals are often used due to their compact architecture. The RF front end of monostatic radar systems typically comprises a directional coupler (e.g., a rat race coupler) for separating the signals to be transmitted from the received signals. A received signal is down-mixed into a baseband or an intermediate frequency band (IF-band) by a mixer which is connected to the directional coupler. The baseband signal or the intermediate frequency signal (IF-signal) being provided at the output of the mixer may be digitized for further digital signal processing.
A real directional coupler, which may be realized using microstrip lines, does not achieve ideal properties with respect to through-loss and isolation, which ideally is zero or infinity, dependent on the pair of ports of the directional coupler. The oscillator signal which is supplied to an input-port of the directional coupler for being transmitted is not only coupled to the port which is connected to the antenna, but a small part of the oscillator signal (which means a fraction of the power of the oscillator signal) is also coupled to the port which is connected to a signal input of the mixer. This part of the oscillator signal is superimposed with the signal received by the antenna at the mixer input. This superimposition results in a DC signal offset at the output of the mixer which is superimposed with the baseband-signal or the IF-signal respectively. Especially when using active mixers this DC signal-offset can be very disturbing. The DC signal offset increases with an increasing transmitting power. Consequently, the DC signal-offset is a parameter limiting the power of the signal to be transmitted and therefore limiting the field of view of the radar sensor.
This effect is not limited to radar applications, but can also occur in general communication applications. There is a general need to provide an RF front-end with a directional coupler and a mixer, wherein the DC signal-offset at the mixer output is substantially eliminated.